Fibular displacement does not predict instability in type B ankle fractures.

Despite the wide prevalence of ankle fractures, no consensus exists on the most accurate radiologic diagnostic tool to indicate medial ligament injury in isolated type B distal fibular fractures. The aim of this study was to evaluate the value of the fibular fracture displacement in predicting medial clear space widening on the gravity stress radiographs, as a parameter of fracture instability. This retrospective cohort study included 192 patients with an isolated type B fibular fracture, for which a regular mortise and gravity stress radiograph were made in our hospital between January 2014 and December 2019. On the regular mortise and lateral radiographs, the medial clear space (MCS), superior clear space (SCS), anteroposterior and lateral fibular displacement were measured. On the gravity stress radiograph, the MCS and SCS were measured. Instability was defined as MCS ≥ SCS + 3.0 mm on the gravity stress radiograph. A receiver operating characteristic (ROC) curve was constructed to evaluate the predictive value of the fibular displacement. Of the 192 included patients, 55 (29%) patients had instable ankle fractures. In predicting instability, fibular displacement demonstrated an area under the curve (AUC) of 0.68 (95% confidence interval 0.60–0.77) and a correlation coefficient of 0.41 with MCS. Fibular displacement on regular mortise view is a poor predictor of instability in type B fibular fractures. It should not be advised to use the fracture displacement as parameter for medial injury and thus for operative treatment.